Yet our study of illusions would not be complete without a glance at this part of the subject.
Hallucination, by which I mean the projection of a mental image outwards when there is no external agency answering to it, assumes one of two fairly distinct forms: it may present itself either as a semblance of an external impression with the minimum amount of interpretation, or as a counterfeit of a completely developed percept.

Thus, a visual hallucination may assume the aspect of a sensation of light or colour which we vaguely refer to a certain region of the external world, or of a vision of some recognizable object.

All of us frequently have incomplete visual and auditory hallucinations of the first order, whereas the complete hallucinations of the second order are comparatively rare.

The first I shall call rudimentary, the second developed, hallucinations.
Rudimentary hallucinations may have either a peripheral or a central origin.

They may first of all have their starting-point in those subjective sensations which, as we have seen, are connected with certain processes set up in the peripheral regions of the nervous system.